Echograms that represented Arctic cod schools as "hollow" masses revealed that acoustic shading was prevalent in our data (see Discussion).
Although the tow-yo sampling pattern mixed the catch across the depth ranges sampled, the samples reflected the zooplankton that occurred where the fish were because sampling was limited to depths where most fish echoes occurred on the echograms. The most common species was the copepod Calanus hyperboreus Kroyer.
When compared with records from all other areas examined, the echograms from MIC were also uncharacteristically devoid of echoes typical of those from zooplankton-sized organisms, reinforcing the netting results.
Next, the [S.sub.v] values in these "speed-filtered" echograms were preliminarily identified as echoes from fish with swim bladders if their sample-wise variance-to-mean ratio (VMR; Demer et al., 2009a) were within the -60 dB to -16 dB range.
For grid cells that did not meet all these criteria, their corresponding [S.sub.v] values in the noise-free echograms were set to -999 dB.
In both corrected and uncorrected echograms
, bottom and near-field exclusion lines were created at 1 m above the bottom and 3 m below the transducer surface.
Detailed examination of the echogram measurements quantified the amount of collinearity, revealed the lack of standardization (subtraction of mean, division by standard deviation) before principal components analysis (PCA), and showed correlations of individual echogram measurements with depth and seafloor slope.
Therefore we conducted an acoustic analysis on data from a small portion from one survey in order to determine if there was a direct correlation between substrate classes or echogram measurements with species abundance.
The IKMT was operated by observation of the DSL in the echogram
, by making an oblique haul for 30 min.